Only execute conditions/reactions that are used by contexts

Only execute conditions/reactions if they are actually used by at least one context on a given site.

Some condition checks (in particular, the menu active trail check) can be very expensive on certain sites. Given that context already caches a map of conditions to contexts, it should be straightforward to check that contexts actually use a given condition before executing its condition.

A solution for this should be pushed into both 2.x and 3.x.